THIRD GREAT DIVISION OF THE ANIMAL KINGDOM. 



THE ARTICULATED ANIMALS. 



This third general type of organization is quite as strongly characterized as that 

 of the Verteb'.ata. The skeleton is not internal, as iu the latter : but is seldom 

 altogether absent, as in the Mollusks. The articulated rings which encircle the body, 

 and frequently the limbs, supply the place of skeleton — and being, in almost every 

 instance, tolerably hard, furnish the necessary resisting fulcra to the muscles of loco- 

 motion ; whence, as among the Vertebrates, we find that the several actions of stepping, 

 running, leaping, swimming, and flying, are performed by them. There are also some 

 famiUes among them that are either footless, or have merely soft and membranous 

 articulated limbs, by which they can at most crawl. This external position of their 

 hard parts, with the muscles inward, reduces each articulation to the condition of a 

 case, and only permits of two kinds of movements. When attached to the next arti- 

 culation by a closed joint, as in the instance of the limbs, the only motion is by 

 ginglymus, that is, in a single direction, so that numerous articulations are required to 

 impart variety of action : and from this results a very great loss of power in the 

 muscles, and consequently a general feebleness in the creature in proportion to its 

 magnitude. The articulated pieces which compose the body frame- work, however, 

 are not always thus connected ; being oftener united by flexible membranes only, 

 which slide considerably one over another, and so allow of more varied movements, 

 but not of the same force. 



The system of organs in which all Articulated Animals bear the nearest resemblance 

 to each other, is that of the nerves. 



Their brain, placed over the oesophagus, and supplying nerves to the parts ad- 

 jacent to the head, is very small. Two chords, which encircle the oesophagus, are 

 continued along the abdomen, and are connected at intervals by double knots or 

 ganglia, from which the nerves of the body and of the limbs are sent forth. Each of 

 these ganglia seems to perform the functions of a brain to the adjoining parts, and 

 continues for a certain time to confer sensibility on them, after the animal has been 

 divided. If to this be added, that the jaws of these animals, whenever they have 

 any, are invariably lateral, and open and shut outward and inward, and not upwards 

 and downwards, and that in none of them has a distinct organ of smell yet been dis- 

 covered, nearly all has been expressed which it seems can be stated of them generally: 

 for the existence of organs of hearing ; the presence, number, and form of those of 

 sight; the productiveness and mode of generation*; their kind of respiration; the ex- 



* A remarkable discovery eoonected with this subject is that of 

 M Herold, who found that in the egg of CrustacQans and Arach- 

 uidcs, the yolk cumiuonicates with the back through the icterior. — 



See his Dissertation on the Eggs of Spiders, Marbourif, 1824 ; aiid 

 thst of M. Rathke on the Et;e» "f Crabs, Leipsic, 1S39. 



